SEDIMENTARY ROCKS TERMS Weathering : breakdown of pre-existing rock Physical or chemical breakdown Erosion: transport of sediment on Earths surface Erosional agents: processes that move sediment SEDIMENTARY ROCKS Introduction Detrital Rocks Chemical/Organic Rocks Classification Lithification Sedimentary Environments Sedimentary Structures Sedimentary Rocks Introduction INTRODUCTION Origin Only form at Earths surface Weathering of pre-existing rock into soluble and insoluble sediments Erosion of sediments Deposition of solid sediment Precipitation of dissolved sediment GEOL 131: Sedimentary Rocks INTRODUCTION Importance Record of past surface conditions Fossils Economically important materials Fossil fuels, gypsum, phosphates, iron, gravel, etc. GEOL 131: Sedimentary Rocks Sedimentary Rocks Detrital Rocks Detrital from detritus - debris Composed of solid sediment grains Distinguished by size of grains GEOL 131: Sedimentary Rocks Detrital rock with large grains Detrital rock with smaller grains Detrital Rocks: Common Minerals Quartz Physically durable Feldspars Physically durable Break down chemically into clays Clay minerals Chemically stable GEOL 131: Sedimentary Rocks Shale Most abundant sedimentary rock 2/3 of all sedimentary rock is shale Made of clay grains Microscopic sheets From chemical weathering of feldspar GEOL 131: Sedimentary Rocks - Detrital Shale GEOL 131: Sedimentary Rocks - Detrital From: From: claysandminerals.com Scanning electron microscope image of shale showing flat clay grains. Scale bar is cm. Shale Formation Quiet environment Burial and compaction of clay grains Not cemented Weak GEOL 131: Sedimentary Rocks - Detrital From: From: geoexpro.com Sandstone Cemented sand-size grains Sand: 1/16mm 2mm diameter Quartz is predominant mineral GEOL 131: Sedimentary Rocks - Detrital Sandstone: Sorting and Shape GEOL 131: Sedimentary Rocks - Detrital Sandstone: Sorting and Shape What controls sorting & shape? Sorting: Erosional agent, transport distance Angularity: Erosional agent, transport distance Sphericity: Mineral types present GEOL 131: Sedimentary Rocks - Detrital Sandstone: Sorting, Shape, & Erosional Agents GEOL 131: Sedimentary Rocks - Detrital WELL SORTED ROUNDED ANGULAR POORLY SORTED WIND GLACIER, GRAVITY WAVES RIVERS Sandstone: Sorting, Shape, & Transport Time - Wind GEOL 131: Sedimentary Rocks - Detrital WELL SORTED ROUNDED ANGULAR POORLY SORTED VERY SHORT (MINUTES TO MONTHS) VERY LONG (MILLENNIA) SHORT/MODERATE (WEEKS TO YEARS) Sandstone: Sorting, Shape, & Transport Time - Glacier GEOL 131: Sedimentary Rocks - Detrital WELL SORTED ROUNDED ANGULAR POORLY SORTED VERY SHORT (WEEKS TO MONTHS) VERY LONG (MILLENNIA) CONGLOMERATE/BRECCIA Cemented gravel-size grains Gravel: greater than 2mm GEOL 131: Sedimentary Rocks - Detrital CONGLOMERATE has rounded gravelBRECCIA has angular gravel Sedimentary Rocks Chemical Rocks Dissolved sediment precipitates out of solution as crystals Inorganic processes: e.g. evaporation, chemical activity Biochemical processes, e.g. growth of shells GEOL 131: Sedimentary Rocks From: rogerdhansen.wordpress.com Bonneville Salt Flats, Utah Chemical Rocks: Limestones Composed mainly of calcite (CaCO 3 ) Several kinds Some biochemical and some inorganic Most form in marine environments GEOL 131: Sedimentary Rocks - Chemical Biochemical Limestones GEOL 131: Sedimentary Rocks - Chemical From:From: seashellsbymillhill.com From: proprofs.com Fossil limestone -Shells + matrix -Hard Coquina -Shells only -Fragile Chalk -Microscopic plankton shells -Very soft Biochemical Limestones GEOL 131: Sedimentary Rocks - Chemical Chalk under a scanning electron microscope, showing shells (round) and calcite crystals. Scale bar is cm. From: From: greenelectron-images.co.uk Inorganic Limestones GEOL 131: Sedimentary Rocks - Chemical Micrite -Microscopic calcite crystals -Precipitated from oversaturated seawater Travertine -Cave rock -Porous Oolitic limestone -Round ooids + cement -Precipitation of crystals from seawater around a nucleus -Rolling back and forth From: www4.uwm.edu From: russianriverschoolhouse.com From: Inorganic Limestones: Ooids GEOL 131: Sedimentary Rocks - Chemical -Ooids form when calcite precipitates from oversaturated seawater onto nuclei being rolled around on seafloor by currents -Nucleus can be any small solid particle: sand grain, piece of shell, etc. Seafloor Nucleus Calcite layers Ooid cross-section Ooid forming Chemical Rocks: Dolostone Composed mainly of dolomite: CaMg(CO 3 ) 2 Origin not well understood Most dolostone is probably converted from limestone when Mg is added Mg-rich groundwater? Seawater? GEOL 131: Sedimentary Rocks - Chemical Chemical Rocks: Chert Biochemical Composed mainly of microcrystalline quartz Very hard; sharp edges Made from compacted microscopic plankton shells GEOL 131: Sedimentary Rocks - Chemical Chemical Rocks: Evaporites Inorganic Evaporation of water (usually salt water) Rock salt, rock gypsum GEOL 131: Sedimentary Rocks - Chemical Bonneville Salt Flats, the ancient floor of Lake Bonneville, a salty lake now mostly evaporated Chemical Rocks: Coal Organic, no minerals Chemically altered dead plant matter Need oxygen- poor water GEOL 131: Sedimentary Rocks - Chemical From: eis.uow.edu.au Sedimentary Rocks Classification Classification: Detrital GEOL 131: Sedimentary Rocks Classification: Chemical GEOL 131: Sedimentary Rocks Sedimentary Rocks Lithification How sediment becomes rock Lith = stone GEOL 131: Sedimentary Rocks - Lithification Lithification Processes Compaction Cementation GEOL 131: Sedimentary Rocks - Lithification Lithification Begins with deposition & burial of sediment under more sediment GEOL 131: Sedimentary Rocks - Lithification Compaction Decreases porosity (empty space) Drives out water Can make flat grains stick together GEOL 131: Sedimentary Rocks - Lithification New crystals form between grains Precipitation out of groundwater Fills in remaining porosity Cementation GEOL 131: Sedimentary Rocks - Lithification From: unibe.ch From: northstonematerials.com Original sand grain Cement crystals Sedimentary Rocks Sedimentary Environments Any place where sediments accumulate Rocks give clues to past environments Reconstruct what Earths surface looked like in the past GEOL 131: Sedimentary Rocks - Environments Examples of some sedimentary environments From: neiu.edu Categories of Sed. Environments Nonmarine Transitional Marine GEOL 131: Sedimentary Rocks - Environments Sed. Rocks Represent Past Environments GEOL 131: Sedimentary Rocks - Environments UTAH Ancient Lake Bonneville Bonneville Salt Flats (white areas): rock salt deposits represent ancient lake Walthers Law Vertical stack of rock layers represents ancient environments that were horizontally adjacent Example: a receding shoreline GEOL 131: Sedimentary Rocks - Environments Sedimentary Rocks Sedimentary Structures Layers (aka beds or strata) GEOL 131: Sedimentary Rocks - Sedimentary Structures Most common type of structure Bedding planes: contacts between layers Cross-bedding GEOL 131: Sedimentary Rocks - Sedimentary Structures Cross-beds in a modern sand dune Cross-beds in sandstone Cross-bedding shows depositional currents flow direction GEOL 131: Sedimentary Rocks - Sedimentary Structures Wind, river, waves, etc. Paleocurrent: ancient depositional current Flow direction Crossbeds Graded beds GEOL 131: Sedimentary Rocks - Sedimentary Structures Grains are sorted by size within a single layer Ripple marks & mud cracks GEOL 131: Sedimentary Rocks - Sedimentary Structures Ripple marks in sandstone Mud cracks in modern sediment End of Sedimentary Rocks Chapter